1. Field of the Invention
The present invention generally relates to semiconductor lasers, laser controllers, frequency converted light sources, and other optical systems incorporating semiconductor lasers. More specifically, the present invention relates to methods for producing a stable output beam from a frequency converted light source that includes, inter alia, a semiconductor laser optically coupled to a second harmonic generation (SHG) crystal, or another type of wavelength conversion device.
2. Technical Background
Short wavelength frequency converted light sources can be formed by combining a single-wavelength semiconductor laser, such as an infrared or near-infrared distributed feedback (DFB) laser, distributed Bragg reflector (DBR) laser, or Fabry-Perot laser, with a light wavelength conversion device, such as a second harmonic generation (SHG) crystal. Typically, the SHG crystal is used to generate higher harmonic waves of the fundamental beam of the semiconductor laser. To do so, the wavelength of the fundamental beam is preferably tuned to the spectral center of the phase matching band of the wavelength converting SHG crystal and the fundamental beam is preferably aligned with the waveguide portion at the input facet of the wavelength converting crystal.
The phase matching band of typical SHG crystals, such as MgO-doped periodically poled lithium niobate (MgO:PPLN) crystals, may have a bandwidth of less than one nanometer. For example, in some MgO:PPLN crystals the phase matching bandwidth may be less than about 0.25 nm. However, the wavelength of the fundamental beam emitted by the semiconductor laser may be tunable over a range of several nanometers. Accordingly, it may be challenging to tune the fundamental beam emitted by the semiconductor laser such that the wavelength of the fundamental beam corresponds to the spectral center of the phase matching band thereby optimizing the light output of the wavelength conversion device.
Tuning the laser to the spectral center of the phase matching band may be further complicated due to thermal fluctuations which occur in the frequency converted light source, particularly during start-up of the frequency converted light source. For example, the laser wavelength tuning mechanism may be thermally coupled to the semiconductor laser such that the tuning signal changes whenever the lasing current and/or laser temperature changes. Further, the spectral center of the phase matching band of the SHG crystal may fluctuate with changes in temperature. Both conditions may cause the power and/or wavelength of light emitted from the wavelength conversion device to vary.
While thermal fluctuations may occur during operation of the frequency converted light source, particularly large thermal fluctuations occur when the frequency converted light source is switched on (e.g., during a cold start). These large thermal fluctuations may last from five to ten seconds during which time it may be difficult to tune the fundamental beam of the semiconductor laser with the spectral center of the phase matching band of the SHG crystal thereby prolonging the start time of the frequency converted light source.
Accordingly, alternative methods are needed for operating a frequency converted light source such that the output beam emitted from the frequency converted light source is rapidly stabilized.